Parting off

I was interested to read comments in the thread on knurling ,( the fear of parting off) I spent a large part of my working life in the small tools business . All the material we used were tool steel , M2 M42 plus carbon chrome and some ASP23 and we parted dia as varied 0.039" to 3 " using hand ground HSS tools , Churchill parting blades and ISCAR carbide tips .This was done on a range of single spindle automatics and later on CNCs , not forgetting early on , lots of Lady's on capston lathes cutting off specials . Now , I have my own lathe in the garage and find parting off quite tricky, you would think with all the parting I have done it would be a walk in the park ,it isn't . I put it down to two or three things ,1 my machine is small and no where near as rigid, 2. My grinder and wheels are of a lesser quality. 3 the material is in most cases not known , I have just got a couple of bits of EN1A and think this will be as easy to cut as brass aluminium etc . So not into football ,think I will go and play in the garage for a while. Stewart.

An interesting story, i would add that simply using the correct geometry, whether you do that yourself on the grinder or it's pre-made on a tip, makes a world of difference to how hard it is to part off, regardless of the machine size.

I would say i've found the thin parting blades to be very bendy and should only be used at the appropriate length. They all come with their own set of limitations though as the insert tip tools need to be inserted properly or they can snap. The most rigid of all will be the solid HSS hand ground tools you often see but you must be careful on the geometry and needs practice to get spot on. When using HSS the speed factor means you can overspeed the steel or it will fray and char.

This is probably the thing that catches out most small machines because the proper torque reduction needs to be applied so that you get the full power of the motor at lets say 500rpm rather than 2000rpm.

This isn't to say that a small motor is bad or wont work it just needs the proper reduction, this is where electronic speed controllers often fail(some more complicated ones have a "compensating factor" )but most machines should have change pulleys for slower speeds.

Perhaps a little courage wouldn't go amiss on parting off either! I've rarely had failures now i've taken these things into account. 

Michael W

Edited By Michael Walters on 10/07/2016 19:57:25

Well having used a mini lathe of late I'd guess it may be a good idea to clamp the saddle and if it happens to face a lot of dish people might still have problems.

They could try flexing the blade side to side a little with the saddle wheel but then clamping is not possible.

I recently bought a 10mm x 2mm HSS blade for the mini lathe but haven't used it yet. I'd guess from other work on it that clamping the saddle will help parting off but for straight turning it will produce a pretty high quality finish so the rigidity can't be that bad. I posted a shot a while ago.

The usual problem though is too slow a feed, fear and keeping an even and yet adequate feed. It's not the end of the world if a blade breaks. Personally I square the blade up against the face of the chuck. That's worked out ok on all lathes I have owned - so far.

I did have problems with an ML7 and at times had to ignore the noise and just carry on as a slight increase in feed didn't get rid of it. It had a rather loose bed and spindle. Most of the problem was the spindle.

John

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Just thinking outloud - If a rear mounted toolpost provides enough flex to overcome all or nearly all parting off issues, could this flex action be built into a QCTP tool on the basis of a high strength compression spring or similar mechanism? This would have been tried by many previously yes, or just dismissed as not do-able?

Ed.

Ed, you might find what you are looking for in the pictures album I have on here. You are looking for the colourful displacement/stress analysis pictures.

Mark

PS. there was a thread on here a while back that looked at parting and how rigid everything was but cant remember when?

i broke another brand new carbide parting tool (brazed, but commercial quality) last week. I think that the major problem in the home workshop,is that we try to push the envelope right to the very edge of what is possible, but I still broke it, and I know why! It broke, because the bar moved in the chuck, because I was making a bush for a pulley on a short end of mild steel, and the remains wasn't long enough to fill the chuck jaws. In industry, this bit of steel would have gone in the scrap bin (and I would have got it out and "repurposed" it in industry, they would have been using a collet chuck, not a three jaw with an ever so slight bell mouth! You need a full tight chuck with a good grip of the material. You need to lock the carriage, you need a properly ground tool, on centre height, in a rigid tool post, in a rigid machine, with a powerful motor and a good deal of "grunt" or you need a tool designed to stop the inevitable "dig in" if something goes wrong, like a rear tool post, upside down tool in reverse, or a swan neck tool holder. Sure, it works in industry, because industry has the time, and the skilled staff to make it work, and ultra high quality rigid machines to do it on. We are not in industry, we need to abide by all the above rules.

The day before my disaster, with the same tool, I had parted of the end of a cast iron pullet about 2" diameter with a 1 1/4"bore. Worked perfectly, no problems, and emboldened by success, I ploughed in to the mild steel with a less than good grip on the bar (and the situation!) I shall repurpose the broken parting tool as an external, 60 degree threading tool, and continue trying to perfect my parting off skills. I will do it....................but I won't enjoy it

Cast iron PULLET!!! it was a pulley, cruelty to chickens I will not stand for!

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Have a look at this Swan-Neck, Ed.

MichaelG.

I have the J&S version of that tool, but without the slit in it, surely the blade spans the slit and when clamped in place, will negate the spring in the tool?

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I can't be 100% certain, but; I think the blade is 'clearance' in the rear section.

Open the second photo in full resolution ... Do you agree?

MichaelG.

I think yer reet an all! It wouldn't make sense if it wasn't, and using a short portion of blade negates the purpose of the design really. Not too difficult to make this type of mod to a standard tool, this has got me thinking, but I mustn't get too far ahead of myself because although there is enough meat on my J&S version to do this mod, the blade in is for a different make, which I have chamfered to fit the holder, again, pushing my luck, as if it moves down at all, the chances are it will grab, and we are back to square one! You certainly can't cut corners with parting off!

Funnily enough, I've not had any trouble parting off ( that's it - I've done it now - next time will be a wrecked parting tool ! ). I use a couple of Glanze 2mm wide throw away tip parting tools - for small diameter stuff, the tool-holder with a maximum capacity of about 25mm diameter and larger, one of the blade type.

I reduce my RPM, then plough straight in and it all seems to work out. With steel or stainless, a few drops of thin oil every few seconds, aluminium a squirt of WD40 and, brass and plastics dry.

I think that confidence has a lot to do with it - we were taught in the training school to 'get stuck in'. I remember parting off rectangular section bar about 50mm x 25mm for some EITB ( Engineering Industry Training Board ) exercise or other ( now I'm going to have to go and look in my apprenticeship made toolbox to find out what we used that rectangular bar for .

dont know about Phil, but yes i do, its a big bit kit though and it takes 31 mm blade, be ok on Phil`s big Cov-Mac, am using 3/32 x 1/2 inch.hss blades, dont have much trouble parting off , but it took awhile to get there all the usual problems one gets with old " used" lathe,- worn bed ( 4 thou dip usual place front shear l/h end ) worn saddle ( 7 1/2 thou difference between l/h front wing and r/h rear saddle wing.) also 3/32 inch difference between l/h and r/h bed risers , that came as a bit a surprise as the lathes originally first owner purchased it new from Myford in 1946-7, He was a regular contribute`r to ME and he did do a write up about the lathe , i think the title of the piece that appeared in the ME was " Myfords new 4 inch centre lathe " or something like that, sometime in the late 1940s i knew it was bit worn when i bought it , but it had some redeeming features anyway i thought i could sort it out , which i did eventually , a new chuck made a big difference when parting off , the mandrel nose adapter ( on the other thread ) was made using new the new chuck ( 41/2 inch Vertex) holding 2 inch steel bar stock parting off was no problem

So yes i think a good chuck goes along way to helping with parting off ops, i wouldnt have said any of the old s/h chucks i have were bell mouthed. But was surprised at difference it made when using the new chuck..................b

Lot of practical discussion of parting with carbide, especially on mini lathes HERE

Neil

I reckon rear mounted parting off tools on a Myford 7 type work better for the simple reason that they reverse the loading one the slides, bed and saddle so if an even feed is used all the bits that can move about are held in place. Mostly because wear occurs in the opposite direction in normal use. Might be different when they ceased using a narrow guide on the front rail but what happens when a dovetail slide is lifted - it tightens automatically. Pressure on the front of the front rail is also increased - the opposite of what happens during normal turning. Most of the need for them was established when they all had front narrow guides. In a nutshell a rear parting off tool on these lathes makes use of guides in a way that avoids normal wear causing problems. The usual reason for them is convenience as they can often just be left in place. I can't see that the holders I have seen on them can flex. In fact they look more rigid than a qctp and could allow a deeper blade to be used.

When people mention grabbing it's probably noticing lots of noise and the tool taking bigger cuts from time to time. That's due to it not cutting cleanly and rubbing instead. Increasing the front clearance on the tool may help with that but grinding back rake on the blade is likely to be superior. If both are done too extremely it may literally grab and pull itself into the work. The principle is exactly the same as grinding tools specifically to take huge cuts using minimum power mentioned on here a while ago - taken too far and that can self feed. HSS will always beat carbide in that respect as carbide can't have such extreme angles.

During my training some people did manage to part off noisily. I suspect it was down to poor sharpening and setting but they were always told to carry on regardless and not to be scared by the process.

Insert blades do have back rake but my main interest in them is finish. My Boxford often doesn't leave the same standard of finish as facing does so I have to hold parts in paper or what ever and take a light facing cut. I don't add back rake to the blades and the lathe is ok like that. Some aren't. It seems that insert blades and tips can reliably leave a decent finish and also go to extreme depths.

John

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Edited By Ajohnw on 11/07/2016 11:10:23

I used to have problems parting off until I put an edge on the front of the blade with the bench grinder. It has made things much easier. I keep two blades : one for parting off and the other (with the flat edge)  for recess cuts. I also keep everything locked down (carriage and topslide). If it doesn't need to move then lock it down.

**LINK**

Edited By Brian John on 11/07/2016 11:42:32

Good example of back rake Brian. Hard to know how much but early tool bit holders often set the tool bit circa 15 degrees, maybe a bit less and due to that for normal work didn't need any back rake grinding on at all. Lots of back rake for a bar turning tool intended for very heavy cuts may not be a good idea though. Pre use of carbide that's when super weld tools came out - hardly any back rake at all.

The front clearance angle tends to push the tool away from the work / prevent it digging in so the 2 angles tend to work together.

It all an odd area really. There is plenty of info about using specific angles for certain materials etc but no explanations of what the angles actually do as far as I am aware. The old one - negative rake for brass has always interested me. I've tried it now and again and it doesn't work at all for me.

We used to get odd 10 min lectures on stuff like this. People were also sometimes given the job of making an angled tool bit holder and making a drift to square the hole up. That lecture started off pointing out that a square isn't far off the shape of a circle and that toolbits only needs holding at the corners. So drill the right size of hole and then hammer the drift into it.

John

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It depends on the grade of Brass, John

... Think about how well a 5-sided cutting broach works on a good clock plate.

MichaelG.

Hi I agree with most of what as been said on the subject of parting off , even the bit about having no fear and making the P/T have it . With regards to P/O with the back slide or position , (if you have only one slide) We would always use the front cross slide so as to make sure the cutting surface or top of the tool was covered with cutting oil. So has to dissipate as much heat as possible . When we were taught to grind parting tools ,we were made to use only the front of the wheel ,this gives a hollow ground effect Which helps with cooling and finish ,and never on the side of the wheel .The reason been, its easy to keep the front off the wheel nice and square with the race , and not good practice to grind on the side anyway . Just one more tip , on the older machines if there was more play than could be removed by adjusting the gib strip ,you could set the centre Hight of the tool a little bit high to account for the downward pressure on the tool , thus getting it on centre whilst cutting . Stewart

Unfortunately Bodge, my big Covmac still sits at the back of what I laughably call "the machine shop" motorless, but loved and polished, as there is not room enough to get the motor on till it is in the "New machine shop" at the other end of the workshop. The new floor is in, and next I need to fit new roofing over the top of the existing roof to stop persistent tiny leaks in the felt flat roof (NEVER use felt flat roofs CRAP!!) The sheets have arrived, and the job is imminent, and then the move is on, because I have to make Covmac swarf before Chris Mills, or I shall never hear the last of it!!